As is known, in the mass production of mechanical workpieces that require machining to the machine tool, a robot brings the workpieces to be machined to a machine tool, where suitable means, of the hydraulic type, take over the workpiece and lock it in position to allow the machining thereof.
When machining has finished the aforementioned means release the machined workpiece, which is once again moved away by the robot.
One type of hydraulic means used for this purpose comprises a hydraulic device consisting essentially of a first cylinder in which a hollow piston is slidably inserted comprising a head and a coaxial rod the distal end of which projects from the cylinder.
The cavity in the piston is open only on the side of the piston head.
In this cavity a helical spring is inserted, one end of which rests at the closed end of the rod while the other end abuts on the inner end of a coaxial peg which starts from the closing element of the corresponding end of the first cylinder and is partly inserted in the same cavity.
The helical spring maintains the piston in its position in which the projection of the relative rod from the first cylinder (corresponding to the home condition of the hydraulic device) is maximal, in which position the piston head leans against an annular shoulder provided in the first cylinder.
By feeding pressurized oil to a first chamber partly delimited by the piston and for the remaining part by the inner surface of the first cylinder, the piston can be made to shift so as to move the rod backwards into the first cylinder.
In order for the backward movement of the rod to take place, the counteracting force of the aforementioned helical spring must be overcome.
Means are also provided to obtain, in addition to the backward movement of the rod, also the rotation thereof by a certain angle (rotation-translation).
These means comprise a pair of balls, partially projecting from relative seats provided in the piston head.
When the rod moves backwards, the piston head forces the balls to travel respective tracks formed symmetrically on the lateral surface of the aforementioned peg.
The pattern of the tracks is such that, when the pressurized oil is fed to the aforementioned first chamber, there is a first phase of backward movement and simultaneous rotation of the piston and therefore of the rod about their axis and a second phase of backward movement only of the rod (without rotation).
The device also comprises a second cylinder designed to receive slidably the first cylinder (which acts as a piston) from the second cylinder projecting outwards the end of the first cylinder from which the aforementioned rod projects, the maximum projection being limited by an end-of-stroke medium.
The second cylinder is fixed to a suitable support arranged in the proximity of the machine tool which must perform the machining of the workpiece. Continuing to feed pressurized oil to the aforementioned first chamber, the first cylinder, if it has not already reached the stroke end, tends to increase its projection from the second cylinder.
At this point it should be noticed that at the outer conical end of the rod is fixed, by means of a ring nut, an element projecting transversely to the rod, commonly called bracket, while another element projecting transversely, or counter-bracket, is fixed to the outer end of the first cylinder.
Therefore, when pressurized oil is fed to the aforementioned first chamber, the result is obtained (thanks to the aforementioned ball means and relative tracks) that the bracket approaches the end of the first cylinder in a first phase and at the same time rotates by an angle which allows the bracket to align with the counter-bracket (rotation-translation), while in a second phase (locking stroke) the bracket only approaches.
If between bracket and counter-bracket has been previously positioned a workpiece (carried by a robot and still retained by it) to be machined on the machine tool, following the feeding of pressurized oil to the aforementioned first chamber the bracket is in contact with the workpiece surface.
If pressurized oil is continuously fed to the first chamber, the further backward movement of the bracket being counteracted by the presence of the workpiece, the first cylinder automatically starts moving outwards, consequently approaching also the counter-bracket to the workpiece.
When also the counter-bracket is in contact with the workpiece, if oil is continuously fed, a pressure increase will be generated and therefore the force exerted by the bracket and counter-bracket on the workpiece.
When a predefined pressure value in the oil is achieved, a sequence valve, external to the device, opens the feeding procedure also to a chamber (hereinafter called third chamber) delimited partly by the inner surface of the second cylinder and for the remaining part by the outer surface of the first cylinder. In such a chamber there is, in contact with said outer surface, an elastic bushing, a part of which, rather thin, which surrounds the first cylinder, is deformed when the oil pressure achieves a value which allows to make it adhere to the surface of the first cylinder so as to develop friction forces that allow locking the first cylinder in position relative to the second cylinder.
On the locked workpiece, all the envisaged machining operations can now be performed.
Once these machining operations have been completed, the oil feeding line has to be drained off (so that in both the aforementioned chambers pressure drops to zero) not only to release the first cylinder, but also to automatically bring (thanks to the action of the aforementioned helical spring) the device back to its home position (maximum rod projection).
It should be noticed that the hydraulic device described above is of the single acting type, i.e. on the side of the piston head opposite to the rod there is a further chamber (which will be called hereinafter second chamber) in communication with the outside by means of a conventional vent.
The known hydraulic device described above, however, has a number of drawbacks.
In particular, since to move the rod backwards the force of the helical spring must be overcome which is enclosed in the cavity of the rod itself, it follows that the oil pressure fed to the first chamber can reach significant values (even 20 bars).
Pressure values of this magnitude cause the sealing gaskets to compress, which significantly increases the friction forces involved, with the result that the force exerted by the bracket and/or counter-bracket on the workpiece to be machined can be not negligible and cause excessive deformation of the workpiece (especially when it comes to thin pieces, which do not have an intrinsic strength), so as to discard the machined workpiece because it does not meet the required tolerances.
Another drawback is due to the fact that the helical spring contained in the hollow rod loses its initial characteristics of elasticity over time; therefore a periodical maintenance should be scheduled for the replacement of the spring, to avoid a reduction of the maximum opening (home position) between bracket and counter-bracket, with the risk that when the workpiece, after its machining, is removed by the robot, the workpiece itself may interfere with the bracket, be damaged and consequently being discarded.
Still a further drawback is due to the fact that the device described above is single-acting, i.e. that the aforementioned second chamber is in communication with the outside.
This communication can generate corrosion phenomena, as well as dirt and/or particles being present at the entrance of this chamber produced by the machining of the workpiece, which can alter the operation of the device.
The ideal situation for this type of device would be that the bracket and counter-bracket just brush the piece to be retained before the first cylinder is locked in position relative to the second cylinder, to prevent the forces exerted on the workpiece from deforming it, especially if the workpiece is not intrinsically strong.
To overcome at least partly the above mentioned drawbacks, the patent document IT 1391930 provides a hydraulic device for locking workpieces to be machined which is devoid of the aforementioned helical spring and which, due to the automatic return to the home condition, provides a suitable hydraulic operating medium.
In practice, instead of putting the aforementioned second chamber in communication with the outside (through a vent), in the device shown in patent document IT 1391930 an equivalent chamber (vent-free) is provided to which pressurized oil can be fed in order to bring the device back to its home condition.
A tight-sealing device is thus obtained with a double-acting hydraulic operation, in which the piston movement can be carried out with an oil pressure (for example 5 bar) which is considerably less than that (e.g. 20 bar) required by the known single-acting devices (in which the force must be overcome of the helical spring which counteracts the backward movement of the rod).
The device shown in IT 1391930 is also susceptible of further improvements.
It is noticed, in fact, that the reduction of the oil pressure inside the device causes the seals subjected to a lower pressure to generate considerably lower friction forces than known single-acting devices, but in any case of still not entirely negligible magnitude.
Such residual friction forces, in fact, counteract a resistance to the reciprocal shift of the rod relative to the first cylinder and of the first cylinder relative to the second cylinder and, seen that the movement of the counter-bracket fundamentally takes place when the presence of the workpiece prevents the bracket from further moving backwards, then it is easy to understand that the residual friction forces discharge mechanically on the workpiece in the form of reaction forces, with the risk of shifting/deforming it only for a few hundredths of a millimeter.
To these reaction forces induced by friction forces is added, if necessary, also the component of the weight force of the piston and of the first cylinder that, if e.g. arranged with a vertical axis, hang from the workpiece charging it with a force directed downwards.